Preheat oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.

In a large bowl, combine the yellow cake mix, melted butter, egg, and sweetened condensed milk. Mix until well combined.

Stir in the toffee bits and chocolate chips (if using).

Pour the batter into the prepared baking pan and spread evenly.

Bake for 20-25 minutes, or until a toothpick inserted into the center comes out clean.

Let the bars cool completely in the pan before cutting into squares.

Once cooled, cut into 24 bars and serve.